The present invention relates to a primer composition for adhering a fluororesin coating onto a metal surface, and a method for coating a metal surface with a fluororesin, using the primer composition.
Because of its excellent properties in chemical resistance, heat resistance, non-stickiness, and the like, fluororesins are used as preferred coating materials for metal surfaces, for example, in applications which include linings for chemical units, which are required to be corrosion resistant: linings for rice cookers, and cooking utensils that are required to be corrosion resistant and non-sticky. However, the excellent non-stickiness results in insufficient adhesion to the metal surface, and a variety of methods have been used up to now for improving the adhesion to metal surfaces.
When coating a metal surface with a fluororesin, powder coating is normally carried out for coating the fluororesin, in that a thicker application can be made compared to that of a fluororesin coating made by spray coating, so as to give good corrosion resistance and excellent non-stickiness to the coated surface, as well as providing resistance to the formation of pinholes reaching as deep as the substrate, however, this approach still does not solve the problem of resistance to sticking to the substrate due to the non-stickiness which characterizes the fluororesin, so that powder coating of the metal surface with a fluororesin calls for using, in addition to the above inorganic acid primer, a primer for the fluororesin powder coating containing organic adhesives, such as polyamideimides, polyimides, polyether sulfones, epoxy resins, and the like, followed by powder coating a fluororesin. However, none of these processes provides optimum adhesion and corrosion resistance when used for a primer for a fluororesin powder coating. That is, a thick powder coating application results in a coated film with a large internal stress resulting in the deficiencies of cohesive failure of the primer and a layer-layer delamination between the primer and top coat (powder coating), problems which remain unsolved.
Thermoplastic fluororesins which are film-forming fluororesins, such as tetrafluoroethylene/perfluoroalkyl vinyl ether copolymers (PFA), tetrafluoroethylene/hexafluoropropylene copolymers (FEP), and the like, are capable of exhibiting fluidity at or above their melting points and of adhering to metals with an adhesion strength too weak to be of any practical use. Thus, the conventional approach has been to chemically or physically roughen the metal surface, followed by a thermal fusion or adhering with the intermediary of an adhesive or primer between the fluororesin film and the metal. These procedures, while exhibiting satisfactory initial adhesion strength, have had low heat resistance making it difficult to maintain adhesion strength in service above 200.degree. C., due to the thermal degradation and thermal decomposition of the adhesive itself or decay of the anchoring effect. Thus, it has been difficult to adhere a fluororesin film to metal, and if any adhesion was provided at all, it was of a weak adhesion strength or had undesirable heat resistance.
Primers used as such adhesives have contained such materials as polyamideimides (PAI), polimides (PI), polyphenylene sulfones (PPS), polyether sulfones (PES), and mica, such as in EP 343015--Sumitomo Electric, and Japanese Kokai 58(83)-19702. However, none of the prior art seems to have found the best proportions of the right ingredients for optimum primer to be used with PFA powder coats or film laminating.
The coating of a metal surface, especially for cookware, with a fluororesin by powder coating the metal surface with a fluororesin or adhering a fluororesin film to the metal surface requires assuring secure adhered surfaces without treating the metal surface with an chromic acid or similar inorganic acids that raise toxicity questions. Also needed is improved adhesion between the metal surface and the fluororesin, good heat resistance, corrosion resistance, and durability.